alpha(1)-adrenergic-receptor responsiveness in skeletal muscle during dynamic exercise

Attenuation of sympathetic vasoconstriction (sympatholysis) in working musc les during dynamic exercise is controversial. One potential mechanism is a reduction in alpha(1)-adrenergic-receptor responsiveness. The purpose of th is study was to examine alpha(1)-adrenergic-receptor-mediated vasoconstrict ion in resting and working skeletal muscles by using intra-arterial infusio ns of a selective agonist. Seven mongrel dogs were instrumented chronically with flow probes on the external iliac arteries of both hindlimbs and a ca theter in one femoral artery. A selective alpha(1)-adrenergic-receptor agon ist (phenylephrine) was infused as a bolus into the femoral artery catheter at rest and during exercise. All dogs ran on a motorized treadmill at two exercise intensities (3 and 6 miles/h). Intra-arterial infusions of the sam e effective concentration of phenylephrine elicited reductions in vascular conductance of 76 +/- 4, 76 +/- 6, and 67 +/- 5% (P > 0.05) at rest, 3 mile s/h, and 6 miles/h, respectively. Systemic blood pressure and blood flow in the contralateral iliac artery were unaffected by phenylephrine. These res ults do not demonstrate an attenuation of vasoconstriction to a selective a lpha(1)-agonist during exercise and do not support the concept of sympathol ysis.